In suspected carbon monoxide poisoning, should I order a carboxyhemoglobin level measured by co‑oximetry in the laboratory rather than refer the patient to pulmonary medicine?

Order Carboxyhemoglobin Level in Your Laboratory—No Pulmonology Referral Needed

You should order a carboxyhemoglobin (COHb) level measured by co-oximetry in your own laboratory immediately; this does not require pulmonology consultation or referral. 1

Why This is a Laboratory Test, Not a Specialty Referral

Direct Laboratory Measurement

• COHb measurement is performed by laboratory spectrophotometry using a CO-oximeter, which transilluminates a blood specimen with multiple wavelengths of light and calculates hemoglobin species concentrations. 1
• Either arterial or venous blood can be used, as COHb levels are similar in both. 1
• Many newer blood gas machines incorporate CO-oximeters and perform spectrophotometry directly on injected blood, measuring oxy-, deoxy-, carboxy-, and methemoglobin concentrations. 1

Immediate Availability

• This is a standard emergency department laboratory test that should be available at your facility without specialty consultation. 1
• The test confirms diagnosis while you are already administering 100% oxygen—no delay for referral is appropriate. 1

Critical Action: Start Oxygen BEFORE Lab Results

Do Not Wait for Confirmation

• Administer 100% normobaric oxygen immediately to any patient with suspected CO poisoning while waiting for COHb measurement. 1
• Oxygen accelerates COHb elimination and alleviates tissue hypoxia—this is front-line treatment. 1
• Never delay oxygen therapy awaiting laboratory confirmation. 2

Important Diagnostic Pitfalls

Standard Pulse Oximetry is Unreliable

• Standard pulse oximeters cannot differentiate carboxyhemoglobin from oxyhemoglobin, resulting in falsely reassuring SpO₂ readings >90% even when COHb ≥25%. 1, 2, 3
• This creates a dangerous false sense of security—you must order laboratory co-oximetry. 2, 4

Older Blood Gas Analyzers May Mislead

• Blood gas machines without CO-oximeter capability calculate oxygen saturation from PaO₂ and pH alone, ignoring COHb presence. 1
• A patient with 40% COHb and PaO₂ 100 mmHg would show calculated SaO₂ of 97-98%, when true oxygen-carrying capacity is only 60%. 1
• Ensure your blood gas analyzer has CO-oximetry capability or send a separate specimen for laboratory spectrophotometry. 1

When Fingertip Pulse CO-Oximetry Requires Laboratory Confirmation

Field Measurements Need Verification

• If fingertip pulse CO-oximetry was used at the scene, laboratory-based spectrophotometry confirmation is recommended upon emergency department arrival, especially for patients being considered for hyperbaric oxygen therapy. 1
• The accuracy and reliability of pulse CO-oximeters in clinical settings remain questioned. 1, 5

Understanding COHb Levels

Diagnostic Thresholds

• In nonsmokers, COHb ≥3-4% confirms exposure beyond normal physiological ranges. 2
• In smokers, COHb ≥10% confirms significant exposure (baseline typically 3-5%, increasing ~2.5% per pack/day). 1, 2

Critical Limitation: Levels Don't Predict Severity

• COHb concentrations do not correlate with patient symptoms, clinical presentation, or outcomes. 2, 4
• The test serves only to confirm exposure and document poisoning—not to assess severity or guide treatment intensity. 1, 2
• Low or normal COHb does not exclude clinically significant toxicity if significant time elapsed since exposure or oxygen therapy already started. 1, 2

No Role for Pulmonology in Acute Diagnosis

• CO poisoning diagnosis and initial management are emergency medicine responsibilities—laboratory testing, oxygen administration, and hyperbaric oxygen consideration do not require pulmonology consultation. 1, 6, 7, 8
• Pulmonology referral would only delay critical diagnosis and treatment initiation. 1